The present invention relates to a shielded printed circuit board for mounting integrated circuits which are sensitive to external magnetic fields.
Recently, very high-density magnetic memories, such as magnetic random access memories (MRAMs), have been proposed as storage devices for digital systems. Magnetic random access memories employ one or more ferromagnetic films as storage elements. A typical multilayer-film MRAM includes a plurality of bit or digit lines intersected by a plurality of word lines. At each intersection, a ferromagnetic memory element is interposed between the corresponding bit line and word line to form a storage cell.
When in use, an MRAM cell stores information as digital bits, the logic value of which depends on the states of magnetization of the thin magnetic multilayer films forming each memory cell. As such, the MRAM cell has two stable magnetic configurations, high resistance representing, for example, a logic state 0 and low resistance representing, for example, a logic state 1. The magnetization configurations of the MRAMs depend in turn on the magnetization vectors which are oriented as a result of electromagnetic fields applied to the memory cells. The electromagnetic fields used to read and write data are generated by associated CMOS circuitry. However, stray magnetic fields, which are generated external to the MRAM, may cause errors in memory cell operation when they have sufficient magnitude.
Very high-density MRAMs are particularly sensitive to stray magnetic fields mainly because the minuscule MRAM cells require relatively low magnetic fields which generate magnetic vectors used in read/write operations. These magnetic vectors are, in turn, easily affected and have a magnetic orientation which can be changed by such external stray magnetic fields.
To diminish the negative effects of the stray magnetic fields and to avoid lo sensitivity of MRAM devices to stray magnetic fields, the semiconductor industry could produce memory cells requiring higher switching electromagnetic fields than a stray field which the memory cells would typically encounter. However, the current requirements for operating such memory cells is greatly increased because higher internal fields necessitate more current. Thus, both the reliability and scalability of such high current devices decrease accordingly, and the use of MRAMs which may be affected by stray magnetic fields becomes undesirable.
Accordingly, there is a need for an improved magnetic memory shielding which reduces the effects of external magnetic fields on internal memory cell structures and operations, as well as a method for fabricating such magnetic shielding. There is further a need for minimizing the cost of shielding a magnetic random access memory IC chip from external magnetic fields.
The present invention provides a method and apparatus which provide a shielding structure for a printed circuit board supporting magnetic field sensitive integrated circuits, such as MRAMs, which shield such circuits from external magnetic fields. The invention employs one or more magnetic shields, preferably formed of non-conductive magnetic oxides, which are on, or embedded within, a printed circuit board and, optionally, an additional magnetic shielding structure which surrounds and covers integrated circuit chips mounted on the printed circuit board.